Aspartic acid, carbon atom reactions

The perhydroxy radical formed on the y-carbon atom (Reactions 30 and 31) is a likely precursor of aspartic acid, which we have found in yields of G = 1.1 when PGA was irradiated in 0.1% solution in 02. Further oxidation and a decarboxylation step would be required to give aspartic acid. However, it is not yet known whether the aspartic acid is formed solely as a result of irradiation it may be formed from a labile precursor during acid hydrolysis of the polymer. Our results differ from those reported by Friedberg and Hayden, who found high yields of aspartic acid in PGA irradiated in the absence of 02 we found only traces of aspartic acid from solutions of PGA irradiated under N2 (Figure 3). Glycine formation was not affected by the presence of 02. 

The atoms of the purine ring are contributed by a number of compounds, including amino acids (aspartic acid, glycine, and glutamine), CO2, and N10-formyltetrahydrofolate (Figure 22.5). The purine ring is constructed by a series of reactions that add the donated carbons and nitrogens to a preformed ribose 5-phosphate. (See p. 145 for a discussion of ribose 5-phosphate synthesis by the HMP pathway.)... 

One of the nitrogen atoms of urea comes from ammonia, the other is transferred from the amino acid aspartate, while the carbon atom comes from C02. Ornithine, an amino acid that is not in the standard set of 20 amino acids and is not found in proteins, is the carrier of these nitrogen and carbon atoms. Five enzymatic reactions are involved in the urea cycle (Fig. 1), the first two of which take place in mitochondria, the other three in the cytosol ... 

AP (apuiinic) site a site, lacking a purine base in DNA, that IS targeted by repair enzymes (10.5) arachidonic acid a fatty acid that contains 20 carbon atoms and 4 double bonds the precursor of prostaglandins and leukotrienes (8.8) aspartate transcarbamo)lase (ATCase) a classic example of an allosteric enzyme that catalyzes an early reaction in pyrimidine biosynthesis (6.5)... 

The first stage in the oxidative degradation of amino acids is the removal of the amino group by one of two main pathways, oxidative deamination or transamination. In transamination the amino group is transferred to the a-carbon atom of a keto acid, usually a-ketogjutarate, resulting in the production of another keto acid and glutamate. The reactions are catalysed by enzymes known as aminotransferases. The reaction for aspartate may be represented as ... 

The basic construction of the mathematical model using simplified metabolic networks to describe the reactions of the citric acid cycle and associated transamination reactions between pyruvate and alanine, oxalacetate and aspartate and a-ketoglutarate and glutamate, and the use of the FACSIMILE program (Chance et al., 1977) to solve the rather large number of simultaneous differential equations generated by the model was the same as previously described (Chance etal., 1983). For the present experiments the model was expanded to include an input flux at the level of succinate to represent propionate metabolism to succinyl-CoA, and a dilution of the aspartate pool to represent net proteolysis. These input fluxes required an output flux of carbon from the citric acid cycle in order to maintain a steady state carbon balance, for which the conversion of malate to pyruvate via malic enzyme was chosen. The model calculates the unknown flux parameters to provide a minimum least squares fit of the C fractional enrichments of specific carbon atoms of metabolic intermediates as measured by C NMR spectroscopy. 

Evidence from several lines of investigation indicated a relationship between L-aspartic acid and L-threonine. Studies with isotopically labeled acetate in yeast and bacteria showed that the distribution of label in the 4 carbon atoms of aspartate was the same as found in threonine. Both aspartate and homoserine were found to suppress incorporation of labeled CO2 into threonine, and the label of aspartate was found to appear in a corresponding position in threonine. Mutants of Neurospora and E. coli were found to use homoserine to form threonine other mutants accumulated homoserine, and in E. coli it was found that aspartate was converted to homoserine. In Lactobacilli threonine was found to minimize an aspartic acid requirement. All of these findings support a scheme of reversible reactions ... 

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